The present invention relates to a process for manufacturing PVC-based composite sheets, and also structures that include such sheets, and in particular structures that have a cellular core and at least one such sheet as reinforcement.
In many industries (automotive, structural engineering, shipbuilding, etc.), the aim is to optimize the mechanical properties/weight ratio of the structures used. Many methods have been developed to achieve this objective and one of the most common ones consists in using a honeycomb cellular structure optionally sandwiched between two sheets called “skins”. By combining this technique with the choice of a lightweight material (a polymer rather than a metal), particularly lightweight structures may be obtained.
Application WO 2006/045723 in the name of the Applicant describes a particularly advantageous structure that has a cellular core comprising a honeycomb structure based on PVC (polyvinyl chloride) filled with PUR (polyurethane) foam and two outer PVC layers. Such a structure not only has good performance with respect to the mechanical properties/weight ratio of the structures, but it may also have an excellent fire resistance by means of an appropriate choice of the compositions of the resins (PVC and PUR formulations classified independently as M1 according to the NFP 92.507 standard). This application presents the use of biaxially-oriented PVC sheets as being advantageous (especially for further improving the aforementioned ratio).
An alternative means of increasing the mechanical properties while keeping the weight the same or of reducing the thickness (and hence the weight) while keeping similar mechanical properties would consist in using composite sheets, i.e. sheets that include a reinforcement. From this point of view, “long” fibrous reinforcements (i.e. fibers which have an average length of the order of cm) are renowned for conferring a more pronounced improvement in the tensile behaviour (modulus and strength measured on sheets of equal thicknesses) than “short” fibrous reinforcements (length of the order of mm), and also an increase in the thermal resistance, a better dimensional stability and a better impact resistance. However, with PVC, which is a polymer that does not melt but which can be gelled (i.e. to form a homogeneous mass where the PVC grains have lost their shape and their identity) under condition of being highly melt-kneaded, the use of such “long” reinforcements is difficult and the length of these reinforcements is in any case substantially reduced at the end of this operation. This drawback (reduction in the fiber length) is furthermore also encountered with meltable crystalline polymers when they are processed by kneading (extrusion for example).
Alternative processes have therefore been proposed in order to be able to incorporate polymers into woven or non-woven fibrous supports, and to thus obtain woven or non-woven fibrous reinforcements/polymer composites. Some of these processes consist of a coating of said supports/reinforcements using a latex or a solution of the polymer, followed by an evaporation of the water or of the solvent. Such a technique is costly from an energy viewpoint and does not often allow a good dispersion of the polymer inside the substrate.
Applications WO 99/22920 and WO 2005/038123 propose an improved process that consists in dispersing polymer powder (dry particles) in a network of fibers or filaments using an alternating electric field and then in moulding the resulting ensemble by heating and pressing to form sheets. This process has the advantage of obtaining a more compact composite where the fibers are well impregnated with polymer, at a lower cost. However, the powders exemplified in these applications are either powders of crystalline polymers, which are easy to melt in the matrix, or powders of thermosetting resins that do not require kneading in order to be used.
The Applicant has surprisingly observed that this process also gave good results with PVC, whereas a priori it was not obvious that PVC distributed in a fibrous network and simply hot pressed would be able to gel.